Open Journal of Forestry, 2015, 5, 48-56 Published Online January 2015 in SciRes. http://www.scirp.org/journal/ojf http://dx.doi.org/10.4236/ojf.2015.51006

How to cite this paper: Fujimoto, K., Mochida, Y., Kikuchi, T., Tabuchi, R., Hirata, Y., & Lihpai, S. (2015). The Relationships among Community Type, Peat Layer Thickness, Belowground Carbon Storage and Habitat Age of Mangrove Forests in Pohnpei Island, Micronesia. Open Journal of Forestry, 5, 48-56. http://dx.doi.org/10.4236/ojf.2015.51006

The Relationships among Community Type, Peat Layer Thickness, Belowground Carbon Storage and Habitat Age of Mangrove Forests in Pohnpei Island, Micronesia Kiyoshi Fujimoto1*, Yukira Mochida2, Takao Kikuchi2, Ryuichi Tabuchi3, Yasumasa Hirata3, Saimon Lihpai4 1Faculty of Policy Studies, Nanzan University, Seto, Japan 2Graduate School of Environmental and Information Sciences, Yokohama National University, Yokohama, Japan 3Forestry and Forest Products Research Institute, Tsukuba, Japan 4Division of Forestry and Marine Enforcement, Department of Land & Natural Resources, Pohnpei State Government, Kolonia, The Federated States of Micronesia Email: *kfuji@nanzan-u.ac.jp Received 25 December 2014; accepted 9 January 2015; published 16 January 2015

Copyright © 2015 by authors and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY). http://creativecommons.org/licenses/by/4.0/

Abstract This paper quantifies the relationships among community type, peat layer thickness and habitat age of the mangrove forests in Pohnpei Island, Micronesia and provides a discussion concerning the primary succession and the belowground carbon storage of the main mangrove community types. The ages of the habitat were estimated from a relationship between the thickness of the mangrove peat layer and the formative period, which was decided by calibrated radiocarbon ages. Mangrove communities in the coral reef type habitat were generally arranged in the following or-der, from seaward to landward: 1) the Rhizophora stylosa or Sonneratia alba community (I or II communities), 2) the typical subunit of the S. alba subcommunity of the Rhizophora apiculata— Bruguiera gymnorrhiza community (III(2)a subunit) and 3) the Xylocarpus granatum subunit of the same subcommunity of the same community (III(2)b subunit). Their habitat ages were esti-mated to be younger than 460 years, between 360 and 1070 years and between 860 and 2300 years, respectively. Based on these results and other evidences such as photosynthetic characte-ristics and pollen analysis derived from the previous studies, the primary succession was inferred to have progressed in the order mentioned above. Belowground stored carbon for the main com-munity types in the coral reef type habitat were estimated to be less than 370 t C ha−1 for the I and the II communities, between 290 and 860 t C ha−1 for the III(2)a subunit and between 700 and

*Corresponding author.

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1850 t C ha−1 for the III(2)b subunit.

Keywords Mangrove Forest, Community Type, Habitat Age, Belowground Carbon Storage, Primary Succession

1. Introduction The mangrove forests on Pohnpei Island, in the Federated States of Micronesia, have not significantly suffered from human impact. This means that a clear zonation of mangrove communities can be found on the Island.

An overview of the mangrove forests in Pohnpei that describes their spatial distribution, total area and timber volume was provided by the USDA Forest Service (e.g. MacLean et al., 1986; Petteys et al., 1986). On the other hand, we have studied formative processes of mangrove habitats to predict the effects of anticipated rapid sea-level rise induced by global warming (e.g. Miyagi & Fujimoto, 1989; Kikuchi, 1995) and also elu-cidated forest structure, dynamics, biomass, productivity, and carbon storage abilities of mangrove forests across different habitat types on five permanent plots of 0.1 to 1 ha (e.g. Fujimoto et al., 1995b, 1999b, 2013; Tabuchi, 2006).

Through these studies, we have accumulated the phytosociological data necessary to classify community types, as well as the geomorphological data necessary to estimate the peat depth and habitat age. We also dis-cussed the successional trends using the data obtained before 1993 to describe the relationship between commu-nity types and peat depth in relation to rise in sea-level (Kikuchi et al., 1999).

This paper clarifies the relationships among community type, thickness of the mangrove peat layer and habitat age using our recent data along with the data used in Kikuchi et al. (1999). In this paper “Habitat age” is defined as the number of years that have elapsed since pioneer mangrove species colonized a site. Although the rela-tionship between mangrove vegetation and site environments such as geomorphological conditions and physio-logical adaptations to physico-chemical gradients has been discussed extensively (e.g. Thom, 1967; Ball, 1988), the ages of habitats with different community types is unknown.

The results of this study will contribute to the discussion on the successional series of mangrove forests in the Pacific region and estimate the spatial distribution of stored belowground carbon as well as aboveground. The relationship between belowground carbon storage and community type was not clarified in our previous study (Fujimoto et al., 1999b), though the general value of the coral reef type habitat was discussed. In this paper we also discuss primary succession and the stored belowground carbon per unit area for each main community type.

2. Study Area and Methods Pohnpei is a high oceanic island of about 35,500 ha that consists of Tertiary volcanic rocks and is surrounded by barrier reefs. Annual rainfall is approximately 5000 to 6000 mm, with no apparent dry season. The annual mean temperature is 26.4˚C and the annual variation is not more than 1˚C from the mean temperature. Mean tidal range at Pohnpei Harbor is about 70 cm, and the maximum spring tidal range is approximately 150 cm.

Mangrove habitats develop mainly on the reef flats fringing the island, although some of them are situated in estuaries (Figure 1). This creates two main habitat types, with the former habitat referred to as the coral reef type and the latter as the estuary type. The substrata of these mangrove habitats mainly consist of peat (Miyagi & Fujimoto, 1989; Fujimoto et al., 1995a, 1995b). Based on phytosociological method, the mangrove forests in Pohnpei have been classified into three community types, along with six subcommunities and five subunits, by Mochida et al. (2006) as follows.

I. Rhizophora stylosa community (1) Typical subcommunity (2) Enhalus acoroides subcommunity (3) Bruguiera gymnorrhiza subcommunity II. Sonneratia alba community

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Figure 1. Map showing the study areas and location of transects.

III. Rhizophora apiculate—Bruguiera gymnorrhiza community (1) Typical subcommunity (2) Sonneratia alba subcommunity

a) Typical subunit; b) Xylocarpus granatum subunit; c) Lumnitzera littorea subunit.

(3) Xylocarpus granatum—Heritiera littoraris subcommunity a) Typical subunit; b) Barringtonia racemosa subunit.

In this paper we use a simple coding system to describe the community types, for example; I(1) describes the typical subcommunity of the R. stylosa community and III(2)a indicates the typical subunit of the S. alba sub-community of the Rhizophora apiculata—Bruguiera gymnorrhiza community.

The relationship among vegetation, sediment and topography was examined for nine transects. Seven tran-sects were situated in the coral reef type habitat and two were in the estuary type habitat (Figure 1). Four tran-sects were along a line of permanent plots established in 1994, 2002 and 2003 (Fujimoto et al., 1995b; Tabuchi, 2006). Transects a to e, h and I, which were examined between 1988 and 1993, have been described in our pre-vious publications (Fujimoto et al., 1995a, 1995b; Miyagi & Fujimoto, 1989; Miyagi et al., 1995). The topogra-phy along the transects was surveyed using a pocket compass with a level. A hiller-type peat sampler was used to examine sediments and to collect the samples for radiocarbon dating. The results were compiled in Figure 2 and Figure 3.

Radiocarbon ages obtained from mangrove peat in our previous studies, as well as from lower tidal-flat depo-sits that were overlain by mangrove peat, were calibrated to calendar year using CALIB 7.0 (Table 1). A stan-dard δ13C value of mangrove plant bodies of −27‰ ± 3‰ (e.g. Fujimoto et al., 1999a, 1999b) was applied to ca-librate for the samples that had not obtained a δ13C value. The calibrated ages were obtained from a 50-year moving-average calibration curve.

The habitat age was then estimated from a resemble equation showing the relationship between the thickness of the mangrove peat and the formative period decided by the calibrated ages.

The stored belowground carbon for each main mangrove community was estimated using the data obtained from Fujimoto et al. (1999b), as well as the relationship between community type and the peat depth.

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Figure 2. Community type, landform and sediments along transects in the coral reef type habitat. (a) Revised Fujimoto et al. (1995b); (b) Revised Fujimoto et al. (1995a); (c) (d) Revised Miyagi & Fujimoto (1989); (e) Revised Miyagi et al. (1995); (f) (g) This study. A: Scale for representing tree height, B: Vertical scale for to represent the topographic and geologic profile.

Figure 3. Community type, landform and sediments along transects in the estuary type habitat. (h) Revised Fujimoto et al. (1995b); (i) Revised Fujimoto et al. (1995a). A: Scale for representing tree height, B: vertical scale to represent the topo-graphic and geologic profile, NF: Natural levee forest, FWSF: Freshwater swamp forest.

3. Results 3.1. Characteristics of Community Types along Transects Of the community types described by Mochida et al. (2006), seven were found along the transects. These com-munity types were:I(1) subcommunity, I(3) subcommunity, II community, III(1) subcommunity, III(2)a subunit, III(2)b subunit and III(2)c subunit (Figure 2 & Figure 3).

The I(1) subcommunity developed along the seaward edge of the coral reef type habitat. It was 15 to 50 m in depth (Figures 2(b)-(f)) and was characterized by a tree height of less than 10 m and a high density of prop roots. The II community rarely appeared along the seaward edge of the coral reef type habitat, which was 10 to 20 m in depth (Figure 2(g)) and was characterized by prostrate trees and a high density of pneumatophores. The III(1) subcommunity appeared behind the I(3) subcommunity (Figure 2(e)). The III(2)a subunit usually

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Table 1. Radiocarbon ages obtained from the mangrove peat and the lower tidal-flat deposit on Pohnpei, Micronesia.

Lob. Code Depth (cm)

Measuredage (14C BP, ± 1σ)

Conventional age

(14C BP, ± 1σ)

2σ calibrated age

(cal BP)

Sampled year (AD)

Formative period (years)

Mean depth (cm)

Accumulation rate

(mm/yr)

IAAA-1021391) 80 - 85 350 ± 20 320 ± 30 460-(390)-310 2010 450 82.5 1.8

NU-3742) 180 - 190 930 ± 80 900 ± 90 960-(820)-680 1990 860 185 2.2

NU-3752) 170 - 180 1950 ± 100 1920 ± 110 2130-(1860)-1590 1990 1900 175 0.9

NU-3762) 93 - 107 190 ± 70 160 ± 85 420-390, 320-(170)-0 1990 210 100 4.8

NU-3793) 270 - 285 2450 ± 80 2420 ± 90 2740-(2500)-2320 1991 2541 277.5 1.1

NU-3803) 130 - 140 1650 ± 90 1620 ± 100 1735-(1520)-1310 1991 1561 135 0.9

NU-3813) 80 - 90 370 ± 70 340 ± 85 530-(390)-270, 200-150, 10-0 1991 431 85 2.0

NU-3824) 90 - 100 270 ± 70 240 ± 85 480-(270)-70, 40-0 1991 311 95 3.1

NU-3834) 155 - 170 1590 ± 120 1560 ± 130 1790-1770, 1765-(1470)-1260 1991 1511 162.5 1.1

NU-9015) 110 - 125 1380 ± 95 1350 ± 110 1500-1450, 1440-(1260)-1030, 1020-1000 1997 1307 117.5 0.9

NU-9025) 193 - 208 1800 ± 95 1770 ± 110 1920-(1690)-1420 1997 1737 200.5 1.2

TH-15146) 175 - 200 1210 ± 100 1180 ± 110 1310-(1110)-910 1988 1148 187.5 1.6

TH-15156) 204 - 219 1860 ± 110 1830 ± 120 2040-(1760)-1500, 1465-1430 1988 1798 211.5 1.2

TH-15176) 125 - 150 1310 ± 100 1280 ± 110 1370-(1190)-960 1988 1228 137.5 1.1

N-64287) 90 - 120 740 ± 80 710 ± 90 810-(660)-525 1992 702 105 1.5

N-64297) 150 - 180 720 ± 80 690 ± 90 780-(650)-520 1992 692 165 2.4

N-64302) 90 - 120 790 ± 80 760 ± 90 900-850, 840-830, 820-(710)-550 1992 752 105 1.4

N-64312) 150 - 180 1170 ± 80 1140 ± 90 1270-(1070)-915 1992 1112 165 1.5 *N-64322) 520 - 550 4870 ± 115 4840 ± 125 5890-5790, 5780-(5570)-5310 1992 - - - *N-64332) 390 - 420 4760 ± 95 4730 ± 110 5690-(5450)-5255, 5180-5060 1992 - - -

**IAAA-1203531) 310 - 320 2580 ± 30 2550 ± 30 2750-(2710)-2690, 2640-2510 2011 - - - **Beta-982865) 330 - 360 2610 ± 60 2570 ± 60 2790-(2640)-2450 1997 - - - **Beta-982875) 450 - 480 3240 ± 60 3200 ± 60 3575-(3430)-3320, 3290-3280 1997 - - - **Beta-982885) 270 - 300 3090 ± 70 3050 ± 70 3410-(3260)-3045 1997 - - -

1)This study, 2)After Fujimoto et al. (1995a), 3)After Miyagi et al. (1995), 4)After Omoto (1997), 5)After Fujimoto et al. (1999b), 6)After Miyagi & Fu-jimoto (1989), 7)Fujimoto et al. (unpublished). *Samples obtained from bottom horizon of a mangrove peat layer in an estuary type mangrove habitat, **Samples obtained from lower tidal-flat deposits. Value in parenthesis in the calibrated age shows a median probability. appeared behind the I and II communities (Figure 2(b), Figure 2(e) & Figure 2(g)). The III(2)b subunit, which occupied the widest area of the mangrove forests in Pohnpei, generally appeared behind the III(2)a subunit in coral reef type habitats (Figures 2(b)-(e)) and in estuary type habitats (Figure 3). The III(2)c subunit generally appeared on the inland side of mangrove forests (Figure 2(e)).

3.2. Relationships between Community Type and the Thickness of the Mangrove Peat Layer

Figure 4 shows the distribution of peat depth under the main community types obtained from the coral reef type habitat. The peat depth of the I(1) subcommunity and the II community was usually less than 50 cm (Figure 2(b), Figures 2(e)-(g) & Figure 4). The I(3) subcommunity stood on about a 55 cm deep of mangrove peat layer, though only three points were measured (Figure 4). While the thickness of the peat layer under the III(2)a sub-unit was between 45 and 133 cm (Figure 2(a), Figure 2(b), Figure 2(e), Figure 2(g) & Figure 4), under the III(2)b subunit it was between 107 and 285 cm (Figures 2(b)-(e) & Figure 4). The peat layer under the III(2)c subunit on Transect c was 138 cm, which was thinner than the III(2)b subunit along the same transect (Figure 2(c) & Figure 4). The peaty layer of the estuary type habitat, which was generally covered by the III(2)b subunit except on river banks, ranged from 300 to 500 cm deep (Figure 3).

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Figure 4. Range of peat depth under the main community types of the coral reef type habitat. Values in the figure show the minimum and maximum habitat ages (years) obtained from the equation in Figure 5.

4. Discussion 4.1. Relationship between the Thickness of Mangrove Peat and the Formative Period In Kosrae Island, Micronesia, which is located about 550 km east of Pohnpei, mangrove forests inhabited estu-aries only before 2100 14C BP (2100 cal BP) and retreated landward between 4100 and 3800 14C BP (between 4500 and 4200 cal BP) because of a rapid sea-level rise (Fujimoto et al., 1996). The sea level reached relatively high stand around 3700 14C BP (4100 cal BP) before the last lower sea-level phase around 2100 14C BP. Man-grove forests expanded on coral reefs by 2100 14C BP with the sea-level fall. Although the sea level has gradu-ally risen over the past 2100 years, the habitats have been maintained by an accumulating of mangrove peat.

In Pohnpei, calibrated ages older than 2500 cal BP have not been obtained from mangrove peat in coral reef type habitats (Table 1 & Figure 2). However, ages around 5500 cal BP and between 3400 and 2600 cal BP were obtained from the bottom horizon of mangrove peat and from the lower tidal-flat deposits overlain by mangrove peat in estuary type habitats (Figure 3), respectively. This evidence suggests that the pattern of sea-level changes and mangrove habitat dynamics on Pohnpei are the same as on Kosrae, and that coral reef type mangrove forests did not exist there before 2500 cal BP, although mangrove forests did inhabit estuaries. Therefore, on Pohnpei, the relationship between the thickness of the mangrove peat and the formative period can only be analyzed for the last 2500 years.

Previous studies have demonstrated that, despite a gradual sea-level rise of 1 to 2 mm yr−1, most mangrove habitats with peat substrates in the Asia-Pacific region have been maintained during the last 2000 years by ac- cumulating peat (Fujimoto et al., 1995a, 1996, 1999a). Therefore, mangrove peat accumulation rates indicating over 2 mm yr−1 are possibly contaminated by younger carbon through processes such as root penetration.

Figure 5 shows the relationship between the sampling depth of mangrove peat for radiocarbon dating and the calibrated age during the last 2500 years. The two outliers, NU-376 and NU-382, were rejected by a Smirnov- Grubbs test (p < 0.05) using EZR downloaded from the web site of Jichi Medical University (http://www.jichi.ac.jp/saitama-sct/SaitamaHP.files/download.html). The peat accumulation rate may have va-ried during the last 2500 years depending on the changes of the rate of relative sea-level rise. However, it is dif-ficult to understand the changes using existing data. Thus, we approximated the relationship between the thick-ness of mangrove peat and the formative period by a linear expression to estimate habitat age.

4.2. Relationship between Community Type, Habitat Age and Succession Mangrove communities in the coral reef type habitat in Pohnpei are generally arranged in the following order, from seaward to landward:1) the I or the II community, 2) the III(2)a subunit and 3) the III(2)b subunit. Using the equation in Figure 5, habitat ages of the I(1) subcommunity, the I(3) subcommunity, the II community,

Ⅰ(1)

Ⅰ(3)

Ⅱ

Ⅲ(2)ａ

Ⅲ(2)ｂ

Ⅲ(2)ｃ

Peat depth (cm)

401

457425

457

360 1065

8572283

0 50 100 150 200 250 300

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Figure 5. Relationship between the thickness of the mangrove peat layer and the formative period in Pohnpei, Micronesia.

the III(2)a subunit and the III(2)b subunit were estimated to be younger than 400 years, between 430 and 460 years, younger than 460 years, between 360 and 1070 years and between 860 and 2280 years, respectively (Figure 4).

These results suggest that the primary succession of mangrove forest in Pohnpei has progressed in the order mentioned above, along with a relative rise in the ground level as a result of peat accumulation. The results also suggest that it took about 400 years and 1000 years for the formation of the III(2)a subunit and the III(2)b sub-unit, respectively.

If the peat accumulation rate has equilibrated with the rate in the rise in sea level, the succession may not have proceeded even if enough time has elapsed. During the last 1000 years, the ground level has relatively risen as a result of peat accumulation; consequently allowing succession to progress, though succession is affected not only by changes in sea level but also various other factors such as seed supply and photo-environment.

Imanishi & Kira (1944) inferred almost the same primary successional series as mentioned above based on the arrangement of community types. However, they placed the S. alba dominant forest, which, judging from their description, seems to include the II community and the III(2)a subunit, between the R. stylosa-R. apiculata forest (corresponding to the I community) and the X. granatum-S. alba-B. gymnorrhiza forest (corresponding to the III(2)b subunit). Kikuchi et al. (1999) also described nearly the same successional trends as mentioned above, though the S. alba community was not recognized.

Studies analyzing different variables, such as forest dynamics, photosynthetic characteristics and pollen anal-ysis, are needed to reinforce the discussion concerning the mangrove successional series. We established per-manent plots in Pohnpei mangrove forests and have monitored forest dynamics since 1994 in order to ascertain the successional characteristics of these forests (Fujimoto et al., 1995b, 2013; Tabuchi, 2006). On the other hand, Kitao et al. (2003) found that the light-saturated electron transport rate was higher in S. alba and R. stylosa > R. apiculata and B. gymnorrhiza > X. granatum. Additionally, the tolerant capacity for photo-inhibition was higher in S. alba and R. stylosa > R. apiculata > B. gymnorrhiza and X. granatum. These results support the succes-sional series mentioned above. Moreover, Yamanaka & Kikuchi (1995) reported that the pollen density of Son-neratia was relatively high throughout the peat layer except immediately at the surface and that the pollen den-sity of Rhizophoraceae gradually increased toward the surface of the peat layer, with a final and rapid increase at the surface. This was found in a boring core obtained from the landward part of Transect i, although it was im-possible to identify the pollen at genus or species level for Rhizophoraceae. Nevertheless, the results are consis-tent with this study’s inference on succession.

The differences of formative initial conditions between the I and the II communities and the formative processes of the III(2)c subunit are not yet clear. Further studies are needed to fully understand these problems.

4.3. Belowground Carbon Storage for the Main Community Types The clear relationship between community type and peat depth enables us to estimate the stored belowground carbon per unit area for each community type using the results of Fujimoto et al. (1999b), a value of 1300 t C ha−1 up to 2 m depth of mangrove peat in the coral reef type habitat in Pohnpei, and the known peat depth for

y = 8.011x

R2 = 0.6329

0

500

1000

1500

2000

2500

3000

0 50 100 150 200 250 300

Form

ativ

e pe

riod

(yea

rs)

Thickness of mangrove peat (cm)

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each community type. The peat depths are less than 57 cm for the Iand the II communities, 45 to 133 cm for the III(2)a subunit and

107 to 285 cm for the III(2)b subunit (Figure 4). A significant difference in stored carbon related to peat depth was not recognized up to a depth of 2 m (Fujimoto et al., 1999b). Therefore, assuming that there is a linear rela-tion between peat depth and stored carbon, the belowground carbon of main community types in the coral reef type habitat are estimated to be less than 370 t C ha−1 for the I and the II communities, between 290 and 860 t C ha−1 for the III(2)a subunit and between 700 and 1850 t C ha−1 for the III(2)b subunit. The stored carbon in the estuary type habitat is estimated at about 2200 t C ha−1. This was found by extrapolating and averaging the data obtained from two boring cores in Fujimoto et al. (1999b), which were 1771 t C ha−1 (365 cm deep) for a river-ward site and 2116 t C ha−1 (347 cm deep) for a landward site. These calculations assume that the mean depth of the peaty layer is 4 m and that the lowest layer in these boring cores continues to a depth of 4 m.

5. Conclusion This study clarified the relationship among community type, peat layer thickness, and habitat age of the man-grove forests in Pohnpei Island, Micronesia using the phytosociological and geomorphological data accumulated since 1988. This result enabled us to estimate the belowground carbon storage of the main mangrove community types and to discuss the primary succession and the approximate time required for the mangrove forests on Pa-cific islands in tropics. Analysis of the high-resolution satellite images such as QuickBird will enable us to esti-mate the spatial distribution of the main mangrove community types and, consequently, to quantify the below-ground carbon storage using the results of this study as well as the aboveground.

Acknowledgements We are grateful to Mr. Herson Anson, a former Chief of Division of Resource Management, Department of Re-source Management & Development, Pohnpei State Government, the Federated States of Micronesia for his kind support and arrangement for our field research and to all the staff of Forestry, Pohnpei State Government for their cooperation in the field research. We are also grateful to Dr. Katherine C. Ewel, a former Research Ecologist with the Institute of Pacific Islands Forestry, USDA Forest Service for her review of our draft manu-script. This study was financed by Grant-in Aids for Scientific Research of the Japan Society for the Promotion of Science (FY 2002-2005, No. 14255011; FY 2010-2012, No. 22255009 and FY 2013-2014, No. 25282084), the Global Environment Research Fund by Ministry of the Environment, Japan (FY 2003-2007, No. S2-2b(1)) and Nanzan University Pache Research Subsidy I-A-2 (FY 2014).

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